1. Field of the Invention
This invention relates to heat exchangers, and in particular, automotive type heat exchangers such as are used for cooling engine and transmission oils, or power steering or brake fluids.
2. Description of the Prior Art
Automotive heat exchangers are used with oils and other automotive fluids that are generally cold and highly viscous upon initial vehicle start-up, especially under cold ambient conditions. Further, modern automotive heat exchangers employ very tiny fluid passages and thin-walled material, to maintain the heat exchangers as small and light in weight as possible. The result is that these heat exchangers can be subjected to very high internal pressures, and flow through the heat exchangers can be blocked or severally restricted until the engine warms up and the fluid systems reach normal operating temperatures. In some cases, the problem is so severe that an engine or a transmission can be starved of lubricating oils and actually fail.
In order to overcome these problems, two approaches have been tried in the past. The first is to use what is sometimes referred to as an active bypass device. This is a bypass valve that is incorporated in the heat exchanger to switch the oil or working fluid flow from the heat exchange circuit to a bypass circuit when the fluid is cold and viscous, and to redirect the fluid back to the heat exchange circuit when the fluid is hot and of normal low viscosity. These bypass valves typically are pressure or temperature activated. An example of a pressure type bypass valve is shown in U.S. Pat. No. No. 4,360,055 issued to Donald J. Frost. This patent shows a spring type flap valve. An example of a temperature type bypass valve is shown in U.S. Pat. No. 4,669,532 issued to Masahiro Tejima et al and this patent shows the use of a bi-metallic strip type valve. Other pressure activated valves, such as spring-loaded popper valves, have been used. Other temperature activated devices employing thermal expansion techniques, such as thermally expanding plugs have also been used. A difficulty with all of these active bypass valve heat exchangers, however, is that they are difficult to manufacture resulting in high costs. Also, they are prone to failure, because they containing moving parts.
The second approach used in the past is what is sometimes referred to as the passive type of bypass. This may be in the form of an external bypass circuit such as a separate tube or channel communicating between the supply and return lines running to and from the heat exchanger. The difficulty with this is that it requires extra tubing which is expensive and prone to leaks and damage. Also, there is very little spare room in modern automotive engine compartments, so there is often not enough room for these external bypass circuits. These latter difficulties can be overcome to some extent by incorporating the bypass tubes into the main heat exchanger structure. However, this interferes with the flow distribution through the heat exchange passages and it reduces the heat transfer efficiency of the heat exchanger to such an extent that it is usually necessary to increase the size of the heat exchanger to maintain heat transfer performance within acceptable limits. Often, it is not possible to increase the size of the heat exchanger because of space limitations inside the engine compartment.